The present invention relates to communications systems, and more particularly, to echo and noise suppression in a bi-directional communications link.
In many communications systems, for example landline and wireless telephone systems, voice signals are often transmitted between two system users via a bi-directional communications link. In such systems, speech of a near-end user is typically detected by a near-end microphone at one end of the communications link and then transmitted over the lin to a far-end loudspeaker for reproduction and presentation to a far-end user. Conversely, speech of the far-end user is detected by a far-end microphone and then transmitted via the communications link to a near-end loudspeaker for reproduction and presentation to the near-end user. At either end of the communications link, loudspeaker output detected by a proximate microphone may be inadvertently transmitted back over the communications link, resulting in what may be unacceptably disruptive feedback, or echo, from a user perspective. Furthermore, if the round-trip gain of a near-end microphone is greater than unity at any aiblq frequency, then the system will tend to "howl" as is well known in the art.
Therefore, in order to avoid transmission of such undesirable echo signals, microphone input should be isolated from loudspeaker output. With a conventional telephone handset, in which the handset microphone is situated close to the user's mouth while the handset speaker essentially covers the user's ear, the requisite isolation is easily achieved. However, as the physical size of portable telephones has decreased, and as hands-free speaker-phones have become more popular, manufacturers have moved toward designs in which a microphone and a loudspeaker may be situated physically close to one another, yet relatively far away from the user. As a result, the need for more sophisticated echo suppression techniques has become paramount in modern systems.
The need is particularly pronounced in the case of hands-free automobile telephones, where the closed vehicular environment can cause multiple reflections of a loudspeaker signal to be coupled back to a high-gain hands-free microphone. Movement of the vehicle and changes in the relative directions and strengths of the user and echo signals, for example as windows are opened and closed or as the user moves his head while driving, further complicate the task of echo suppression in the automobile environment. Additionally, more recently developed digital telephones process speech signals through vocoders which introduce significant signal delays and create non-linear signal distortions. As is well known, these prolonged delays tend to magnify the problem of signal echo from a user perspective, and the additional non-linear distortions can make echo suppression difficult.
Traditionally, echo suppression has been accomplished using echo canceling circuits designed to approximate and remove echo signals from microphone output so that only near-end speech is transmitted over the communications link. These systems are described, for example, in U.S. Pat. No. 5,475,731, which is incorporated herein by reference. While the systems described in the cited reference are generally effective in suppressing echo signals, certain aspects of those systems make them impractical in some contexts. For example, as is described in more detail below, residual echo suppression circuits within such systems may be relatively ineffective when ambient noise arises at the microphone input. Ambient noise is commonplace and may occur, for example, due to road and traffic noise in the case of an automobile telephone. Therefore, it would be advantageous if a system were available in which all of the echo suppression features of the system could function effectively even in the presence of ambient noise.
Additionally, certain aspects of available systems are not well suited for double-talk situations in which a near-end user and a far-end user are speaking simultaneously. For example, because residual echo suppression circuits within available systems may intolerably distort near-end signals from a far-end-user perspective, they are typically deactivated during double-talk situations. By deactivating all or part of the echo suppression features, however, a conventional system may be susceptible to other problems. For example, as is described in more detail below, the echo suppression provided by a conventional system employing a adaptive-filter echo canceler may be insufficient, absent residual echo suppression, due to non-linearities introduced by the components used to process information signals. Thus, it would be advantageous if a system were available in which all of the echo suppression aspects of the system could be used even during double-talk situations. In sum, there is a real need for an improved technique for suppressing echo signals in a two-way communications link.